This invention relates to development apparatus for an electrographic copier/duplicator or the like incorporating an improved ribbon blender for mixing developer material in such apparatus.
Electrographic development apparatus are well known in the art. Such apparatus may include a housing in which developer material is located and a magnetic brush that receives such material from the housing and transports it to a position where part of such material can be transferred to an electrostatic image formed on an insulating surface to thereby develop the image. The developer material may comprise a mixture of carrier particles and smaller toner particles. Some of the toner particles are transferred from the mixture to the electrostatic image during development of the image. Thus there is a need to replenish toner particles in used developer material in the housing. In order to replenish toner particles in used developer material, the used developer material can be delivered from the housing to an end sump along with the addition of fresh toner particles. The materials in the end sump are then mixed and agitated in order to thoroughly mix the fresh toner particles with the used developer material and to effect triboelectric charging of the materials. The resulting toner-replenished developer material is then transported from the end sump to the housing by means of augers or other apparatus so that the development material is again available for transfer of toner to a magnetic brush for development of an electrostatic image. An apparatus of this general type is disclosed in commonly assigned U.S. Pat. No. 4,101,211, issued July 18, 1978 in the name of Walter Kayson and entitled MAGNETIC CURTAIN SEAL FOR DEVELOPMENT APPARATUS. Such development apparatus requires relatively high circulation rates in order to transport toner replenished development material from the end sump along the entire length of the housing of the development station. Such high circulation rates increase power requirements for the development apparatus and decrease the developer life.
The carrier particles of a developer material may comprise permanent magnets. The carrier particles in such a material have a tendency to cling to each other to form clumps and, in general, in the absence of an external field, such materials have a tendency to behave somewhat like wet sand due to the magnetic direction attraction exerted between the particles. Such a material creates special problems in mixing developer material, circulating the material axially along a development station, agitating and shearing the developer to promote tribocharging and then feeding the developer to a magnetic brush. A material having carrier particles of permanent magnets is disclosed in the commonly assigned copending U.S. patent application Ser. No. 548,807, filed Nov. 4, 1983 in the names of E. T. Miskinis et al, and entitled TWO COMPONENT, DRY ELECTROGRAPHIC DEVELOPER COMPOSITIONS CONTAINING HARD MAGNETIC CARRIER PARTICLES AND METHODS FOR USING THE SAME.
Commonly assigned copending U.S. patent application Ser. No. 597,323, filed Apr. 6, 1984, in the names of Brian J. Joseph and Thomas K. Hilbert and entitled ELECTROGRAPHIC DEVELOPMENT APPARATUS HAVING A RIBBON BLENDER discloses a development station that is particularly adapted to handle developer materials having carrier particles that comprise permanent magnets such as disclosed in the before-mentioned copending U.S. patent application Ser. No. 548,807. In U.S. patent application Ser. No. 597,323, the development station of an electrographic copier/duplicator or the like has a sump for receiving a two-component developer material. A ribbon blender is located in the sump and comprises an outer helical ribbon and an inner helical ribbon both of which are concentrically located about the axis of a rotatable shaft. When the shaft is rotated, the ribbons move the developer material back and forth through the sump to agitate and shear the developer and promote tribocharging of the developer material. In one embodiment disclosed in that application, the inner and outer ribbons on one end portion of the shaft has a pitch that is opposite from the inner and outer ribbons on the other end portion of the shaft, and the inner ribbon on each end portion of the shaft has a pitch that is opposite from the outer ribbon on the same end portion of the shaft. When the shaft is rotated the outer ribbons drive the developer material toward the center portion of the ribbon blender and the inner ribbons drive the developer material toward the end portions of the ribbon blender.
While the ribbon blender of the before-mentioned application is suitable for mixing developer materials, including those with hard magnetic carrier particles, it has been found that some problems do occur when using such a ribbon blender. More specifically, axial mixing between the two end portions of the ribbon blender is not entirely satisfactory. In addition, the level of developer material across the length of the blender is not always even. Also, replenishment of toner depleted from the developer material during operation is achieved using a somewhat complex replenishment system.